Tracing should allow configurable client_sampling and random_sampling

[luckyxkw]

Contour only allows us to configure overallSampling rate: contourconfig.go#L817.

It will be more useful if we can configure client_sampling and random_sampling as well. Details below:

According to envoy's document, there are three sampling rate controls tracing:

• Client_sampling: sample rate when x-client-trace-id header is set, default to 100%. This can be used to force generate trace for a specific request
• Random_sampling: this is the random sample rate for all requests, default to 100%.
• Overall_sampling: this sample rate is applied to all requests on top of the client_sampling or random_sampling. This means if we have random_sampling = 1/100, and overall_sampling = 1/100, the final randomly SR will be 1/10000. When overall_sampling is less than 100%, x-client-trace-id header does not guarantee to sample the trace.

I think for most people, it makes more sense to have client_sampling=100%, random_sampling={intended random sampling rate}, overall_sampling=100%

[github-actions]

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